The invention relates to head respiratory equipments of the type comprising a breathing mask, a head harness connected to the mask for quick donning onto the head of a user, and sometimes goggles for protection against smoke.
Quick donning harnesses for breathing masks are known which have a stretchable strap whose ends are connected to the mask, including an element which is inflatable with pressurized gas to stretch the strap to a size sufficient for enabling the user to place the strap over his head and which have manually controlled means enabling to deliver pressurized gas to the element to stretch it and to vent the element for causing the strap, due to the inherent resiliency thereof, to contact the head and to maintain the mask. The pressurized gas is typically oxygen which also feeds a demand regulator with air dilution carried by the mask.
Passenger and business air planes fly at increasingly higher altitudes. Beyond 40,000 feet (about 12,200 meters), the mask user should be immediately provided with pressurized breathable gas upon cabin depressurization. For avoiding gas leaks between the face cover and skin, the harness must then exert a high tension. When the flight conditions are such that the regulations require that the pilot or either pilot wears the mask at all times, such continuous use causes tiredness and discomfort. In addition, since the mask should be usable by all pilots, harnesses are constructed to achieve air tightness of the mask for the smallest head size the tension forces are still more important on large size heads.
In an attempt to solve the problem, harnesses have been proposed which have means for maintaining, in the inflatable element, an intermediate pressure, which is called a comfort pressure. For instance European No. 0,288,391 discloses a harness which, in a particular embodiment, further comprises an aneroid valve which automatically causes complete venting of the inflatable element and consequently a tight application of the mask onto the face, without user's manipulation, upon depressurization. U.S. Pat. No. 5,036,846 also discloses a harness having an inflatable element in which a residual intermediate comfort pressure may be maintained.
The harnesses described in both documents have a shortcoming. They require manual adjustment of the residual pressure in the harness and that pressure varies in dependence of the size of the head of the user for a same application force.
In addition, leaks (caused for instance by porosity of the inflatable element and/or by a lack of air tightness of the valves) frequently cause a progressive decrease of the pressure in the inflatable element and consequently a progressive increase of the force which applies the mask on the face, which requires a periodical re-inflation of the harness by the user for comfort.